Dispersion type thermoplastic fluororesin coating materials containing a polyvinylidene fluoride as the main component and solvent-soluble type thermosetting fluororesin coating materials containing as the main component a copolymer of a fluoroolefin and a vinyl ether or a vinyl ester, have been widely used in recent years in coating material fields e.g. for heavy duty coatings, for buildings or for industry, because of such reasons as corrosion resistance and durability of coating films and that dirt is easily cleaned (e.g. JP-B-60-21667, JP-A-59-102962, JP-A-61-57609).
Further, against environmental pollution problems which are increasing year after year, low-pollution type general powder fluororesin coating materials, in a production of which organic solvent (VOC) emission amount is reduced, have been proposed (e.g. JP-A-1-103670, JP-A-2-60968, JP-A-6-345822, JP-A-7-145332).
In general, a chief material resin used for powder coating materials is solidified by removing a solvent from a resin solution or dispersion by means of e.g. a continuous vacuum solvent-removing apparatus or spray dryer, prior to mixing with other additives in many cases (e.g. JP-A-8-118357). However, by such a method, if the resin stays in the apparatus for removing the solvent and is heated for a long time, gel particles may be partially formed, and a step of e.g. filtrating the gel particles is required, and coating properties and physical properties of the coating film may decrease in some cases.
In a case of producing a powder coating composition, a production process is commonly employed wherein the chief material resin is roughly pulverized after the solvent is removed by the above method, a curing agent and various additives are dry-blended, and the blend is melt mixed, pulverized again and classified. Thus, because of such a multi-stage production process, the production cost is high as compared with solvent type coating materials at any rate. In order to overcome this problem, it has been attempted to directly obtain a powder coating material by preparing a raw material solution having a chief material resin, a curing agent and various additives as raw materials dissolved or dispersed in a solution, and removing the solvent.
For example, JP-A-2000-34426 proposes a process to obtain a powder coating material by mixing a chief material resin used for a general powder coating material such as an acryl resin and additives such as a curing agent for it, and then subjecting the resulting mixture to spray drying or supplying it to a continuous vacuum solvent-removing apparatus to remove volatile components, by using a low boiling point solvent such as methanol.
However, the present inventors have found that there are still problems even when a low boiling point solvent is used as mentioned above, in a case of a fluororesin having curable functional groups, as different from e.g. acryl resins. Namely, one problem is that no adequate solid content concentration as a powder coating material can be achieved by solvent removal by means of spray drying or a continuous vacuum solvent-removing apparatus at such a low temperature, since an unreacted monomer having a relatively high boiling point remains in the solution after polymerization in practice. Further, the other problem is that gel particles are formed similarly to the time of removing the solvent from the chief material resin as mentioned above, even at a temperature at which no curing reaction will take place, if the solvent is removed at a relatively high temperature in order to increase the above solid content concentration.
The present invention is, first, to overcome the above problems in a case of producing a powder coating composition by removing a solvent from a solution or dispersion of a fluororesin having curable functional groups to obtain a chief material resin for the powder coating composition, and then dry-blending coating additives such as a curing agent. Secondly, it is to overcome the above problems in a case of directly producing a powder coating material by preparing a raw material solution having a fluororesin having curable functional groups, a curing agent and various additives dissolved or dispersed in a solvent, and removing the solvent by means of a continuous vacuum solvent-removing apparatus.